PET is used on a large scale for containers in the food industry, especially for beverage bottles. In consequence, large quantities of spent waste containers are produced, which can no longer be returned to the filling process. Methods and plants for recycling these waste containers and process them to obtain again a PET source material suited for use with food, for the production of new containers are already known. Such a recycling plant may be provided directly upstream of a production line for the production of new PET containers, where the recycled product coming from the recycling plant can directly be introduced, for example, into an injection molding machine for the production of preforms as starting product for new PET bottles.
There are recycling plants for PET, which are each designed for a specific type of recycling product. For example, the recycling plant may be designed for the production of so-called flakes or the production of pellets. In principle, flakes are the products from a grinder, which have been cleaned, sorted and decontaminated for use in foodstuffs. Pellets are products from an extruder, where they were homogenized by thermal influence. Both production methods have advantages and disadvantages. The advantages of the flake process lie, for example, in a small energy input, in the fact that no acetaldehyde is formed, that the intrinsic viscosity is not reduced, that possibly contained foreign plastics cannot coalesce, and that drying is possible at low temperatures. Disadvantages of the flake process are that flakes are less suitable for being admixed to newly produced PET (virgin PET) and that foreign plastics can oxidize. The advantages of the pellet process lie in a simple and unproblematic admixture to virgin material, and in the use of a melt filter filtration technique for removing finest particles. The disadvantages of the pellet process lie in a considerably greater energy input, in the risk that acetaldehyde is formed, and that foreign plastics may coalesce, so that no homogenous PET is obtained.
Due to this fact both processes are in use and are employed, depending on the interests of the manufacturer. However, once a recycling plant is installed the user is committed to this process, at least for the service life of the plant.